The long-term goal of the proposed research is to characterize the intrarenal regulation of electrolyte transport. It is proposed to study the hormonal system mineralocorticoids. The role of mineralocorticoids in the regulation of sodium and potassium transport in the papillary collecting duct will be studied. The hypotheses to be tested include: a) the sodium retaining and potassium losing properties of known mineralocorticoids can be dissociated at the level of the papillary collecting duct and b) that newly identified mineralocorticoids can be sodium retaining without stimulating potassium secretion in the papillary collecting duct. These problems will be investigated utilizing micropuncture techniques in the hamster and microanalysis by helium glow photometry. BIBLIOGRAPHIC REFERENCES: Hughes, S.P.F., Davies, D.E., Bassingthwaighte, J.B., Kelly, P.J. and Knox, F.G.: Bone Extraction and Blood Clearance of Diphosphate in the Dog. Am. J. Physiol., in press, 1977. Knox, F.G., Preiss, J., Kim, J.K. and Dousa, T.P.: Mechanisms of Resistance to the Phosphaturic Effect of Parathyroid Hormone in the Hamster. J. Clin. Invest., in press, 1977.